The present invention concerns a method for producing a thin membrane and the membrane structure so obtained. This membrane may be made up of one or more materials, monocrystalline materials in particular. It may be self-supporting or fixed to a supporting substrate to rigidify the structure so obtained.
This type of membrane offers many advantages. By way of example, mention may be made of compliant substrates which may use a membrane of the invention, or any other application in which a thin film is required (silicon film on a glass or plastic substrate). By compliant substrate is meant a structure able to accept the stresses induced by a structure adhering to it, and which may be a layer deposited on a surface of this substrate by heteroepitaxy for example.
FR-A-2 681 472 describes a method for fabricating thin films in semiconductor material. This document discloses that the implanting of a rare gas or of hydrogen into a substrate in semiconductor material is able to create microcavities or microbubbles called xe2x80x9cplateletsxe2x80x9d at a depth close to the average penetration depth of the implanted ions. If this substrate is placed in intimate contact, via its implanted surface, with a stiffener, and if heat treatment is applied at a sufficient temperature, an interaction occurs between the microcavities or platelets leading to separation of the semiconductor substrate into two parts: firstly a thin semiconductor film adhering to the stiffener, and secondly the remainder of the semiconductor substrate. Separation occurs at the site where the microcavities or platelets are present. Heat treatment is such that the interaction between the microcavities or platelets formed by implantation causes the separation of the thin film from the remainder of the substrate. A transfer therefore takes place of a thin film from an initial substrate to a stiffener acting as a support for this thin film.
This method may also be applied to the fabrication of a thin film in solid material other than a semiconductor material (a conductor or dielectric material), whether crystalline or not.
With the methods described in FR-A-2 681 472 (corresponding to U.S. Pat. No. 5,374,564) already cited and FR-A-2 767 416, it is possible to transfer a thin film in homogeneous material or made up of homogeneous or heterogeneous multilayers to a mechanical support also called a stiffener. This transfer is advantageously conducted using heat treatment. However, this heat treatment may be associated with or entirely replaced by mechanical separation, for example by using tensile and/or shearing and/or flexion forces applied in separate or combined manner. Such process is described in FR-A-2 748 851.
It has also been shown that this technique could be used without a stiffener if the implanted ions are located at a sufficient depth to induce fracture over the entire substrate with no blister formation at the implanted surface. In this respect, reference may be made to FR-A-2 738 671 (corresponding to U.S. Pat. No. 5,714,395). In this case, to obtain a fracture, the microcavity zone needs to be at a minimum depth relative to the implanted surface so that the thin film may be sufficiently rigid. This rigidity may also be obtained through the use of one or more layers deposited on the thin layer with fracture.
The documents cited above describe methods with which it is possible to obtain a thin layer of material. This layer may be homogeneous, contain all or part of a microelectronic or optoelectronic component, or even be heterogeneous. By heterogeneous is meant that it may be made up of several elements stacked on top of each other. These stacks of layers are generally obtained by epitaxial growth. With epitaxial growth the problem is raised of compatibility between the different layers. These compatibility problems may for example relate to widely varying mesh parameters leading to dislocations in at least one of the layers. On this account, some structures appear impossible to obtain.
Also, document FR-A-2 738 671 already cited, indicates that implantation must be conducted at an energy such that the ion penetration depth is at least equal to a minimum depth for the film to be rigid. It is indicated that, for silicon, the minimum penetration depth is in the order of 5 xcexcm even 4 xcexcm. This relates to an implanting energy of approximately 500 keV. For silicon carbide, which is a much more rigid material than silicon, the minimum possible thickness of the thin film is in the order of 1 xcexcm. With this method, it is therefore possible to obtain thin films or layers whose thickness is greater than a minimum thickness needed to provide the thin layer with some extent of rigidity. It is indicated that a rigid thin layer is a layer whose mechanical properties are sufficient during the second step (which corresponds to heat and/or mechanical treatment) to avoid the onset of swelling, platelets or platelet burst and hence are sufficient for application of the second step to achieve surface detachment. However, with this method, it is impossible, depending upon the mechanical nature of the required film, to obtain self-supporting thin films using standard commercially available implanters, that is to say using implanters having a maximum implanting energy of 200 keV. For example, it is impossible to obtain a silicon film having a thickness of 4 xcexcm with such energy.
Another problem is raised if it is desired to use a standard implanter (energy less than 200 keV) to transfer a thin film onto an ordinary support, that is to say a support which does not have sufficient rigidity to obtain a stiffening effect. For example, it is not possible to transfer a monocrystalline silicon film onto a flexible support such as a plastic support without using an intermediate support of handle-type such as disclosed in FR-A-2 725 074. However, it would be advantageous to have available a method with which it is possible a overcome the need for this handle-type support and which enables direct transfer of the thin film onto its final support.
The invention provides a solution to the above-cited problems. The solution is put forward to fix two substrates to one another via their implanted surface, implantation being conducted such that the substrate cleavage phenomenon may occur at the implanted zones. It is then possible to obtain a membrane formed by the joining of the two thin layers. This membrane may be transferred onto any type of support (semiconductor, metal, plastic, ceramic) with no condition as to adhering force (strong or weak) between the membrane and the support.
The subject matter of the invention is therefore a method for producing a thin membrane, characterised in that it comprises the following steps:
implanting gas species through one surface of a first substrate and through one surface of a second substrate which, in such substrates, are able to create microcavities delimiting for each substrate a thin layer lying between these microcavities and the implanted surface, the microcavities being able, after their implantation, to cause detachment of the thin layer from its substrate;
assembly of the first substrate onto the second substrate such that the implanted surfaces face one another;
detaching each thin layer from its substrate, the thin layers remaining assembled together to provide said thin membrane.
By gas species is meant elements, hydrogen or rare gases for example, in their atomic form (H for example) or in their molecular form (H2 for example) or in their ionic form (H+, H+2. . . for example) or in their isotopic form (deuterium for example) or in isotopic and ionic form.
Also, by implantation is meant any means of inserting the previously defined species, either alone or in combination, such as ion bombardment, diffusion, etc.
According to one variant of embodiment, the steps are conducted in the following order:
implanting the first substrate and the second substrate,
assembly of the first substrate onto the second substrate via the implanted surfaces,
detaching each thin layer, either simultaneously or in succession.
According to a second variant of embodiment, the steps are conducted in the following order:
implanting the first substrate,
assembly of the implanted surface of the first substrate onto a surface of the second substrate intended to undergo subsequent implanting,
detaching the thin layer from the first substrate, which layer remains assembled to the second substrate,
implanting the second substrate through the thin layer detached from the first substrate,
detaching the thin layer from the second substrate, which layer remains assembled to the thin layer of the first substrate to provide said thin membrane.
In both substrates, a preliminary step may be provided prior to the implanting and assembly of the two substrates, consisting of making a layer of inclusions in the substrate to be implanted, at a depth corresponding to the required thickness for the thin layer to be made in this substrate, the inclusions forming traps for the gas species which are to be implanted, for example by ion implantation or diffusion. The layer of inclusions may be formed using a film depositing technique. It may consist of generating lines or generating grain joints.
The average implantation depth of the gas species, in a substrate in monocrystalline material, may be determined in relation to the arrangement of the crystallographic network of the monocrystalline material relative to the direction of implantation. For a given energy, greater average depths of penetration of gas species can be achieved if channelling of the ions or implanted species is used. For this purpose, it suffices to conduct implantation parallel to a crystallographic direction or plane (with monocrystalline material only). In contrary manner, it is possible to reduce the implantation depth at a given energy by conducting inclined implanting. In this case, the crystallographic axis of the material is oriented such that there is no preferential direction of ion penetration in the material.
Optionally, implantation is conducted through one surface of the first substrate and/or of the second substrate from which all or part of at least one electronic and/or optoelectronic and/or optic component and/or microsystem has been fabricated. Implantation may be effective even for masked areas.
According to another embodiment, if detachment of each thin layer is made in successive manner, after a first thin layer detachment has been made, all or part of at least one electronic and/or optoelectronic and/or optic component and/or microsystem is fabricated on the thin layer uncovered by this first detachment. By way of example, the component made in one thin layer is a DRAM memory. If there is surface topology, this surface may be planarized before contacting.
The assembly of the first substrate onto the second substrate may be made using a technique chosen from among bonding by molecular adhesion, bonding by means of an adhesive substance and use of an intermediate compound.
This assembly of the first substrate onto the second substrate may also be made by inserting an intermediate layer. The presence of an intermediate layer may modify the apparent rigidity of the membrane and modify transfer conditions. In particular, it may modify the annealing conditions and/or the mechanical fracture conditions.
Advantageously, the detachment of said thin layers is made by applying heat treatment and/or by applying mechanical forces. In all cases, the transfer conditions evidently depend upon implantation conditions (dose, energy, heat schedule supplied to wafer) and the stresses placed by the structure on the implanted zone. The mechanical forces may comprise tensile and/or shearing and/or flexion forces. The mechanical forces may be applied perpendicular to the planes of the layers and/or parallel thereto. They may be restricted to one point or one zone or applied to different sites in symmetric or asymmetric fashion. The heat energy supplied may for example be supplied using a laser beam. The mechanical energy applied may be applied using ultrasound.
If the detachment of a thin layer implies the application of heat treatment, the latter may be applied under controlled pressure (for example a gas or mechanical pressure). Lowering of the pressure at the time of heat treatment or at the time of separation may facilitate separation. In this way, it is possible to obtain separations at lower doses of implanted gas and/or lower heat treatments. By lower heat treatments is meant annealing conducted at a lower temperature and/or for a shorter time period. If pressure is increased, the fracturing conditions at the time separation occurs are modified and separation can be delayed. This delay may be advantageous in that it induces less surface roughness after fracturing but it may also enable fracturing to be obtained under conditions in which platelet formation is achieved with annealing at atmospheric pressure.
In relation to applications, after detachment from at least one substrate, the thin membrane may be fixed to a final or temporary support.
A further purpose of the invention is a thin membrane structure obtained using the above method. This structure may comprise a support carrying this membrane, this support possibly being in a material chosen from among semiconductor materials, plastic materials, ceramic materials, and transparent materials.
One of the thin layers may be in silicon and the other in III-V semiconductor material, GaAs for example.
The membrane may also comprise an intermediate layer inserted between the two thin layers. For example, the two thin layers are in Si and the intermediate layer is in SiO2, in Si3N4 or in a combination of several materials and/or multilayers. According to another example, the two thin layers are in semiconductor material and the intermediate layer is in a conductor material such as palladium.
According to another variant of embodiment, one of the layers is in Si, the other layer is in Ge, the thin layers being doped so that the structure forms a photovoltaic cell.